PROJECT SUMMARY Chronic obstructive pulmonary disease (COPD) is the fourth leading cause of death in the United States and has no cure. While the association between chronic tobacco smoke exposure and COPD has been known for decades, the reasons for ongoing lung damage in COPD after smoking cessation remain undefined. We and others have shown that epithelial remodeling with loss of multiciliated cells (MCCs) is widespread in the airways of COPD patients and abnormal epithelial differentiation in individual small airways is strongly associated with loss of the secretory IgA (SIgA) barrier, chronic inflammation, and fibrotic remodeling of the airway wall. Further, we showed that loss of SIgA in the airways is sufficient to induce a COPD-like phenotype in mice, suggesting altered mucosal immunity plays a causal role in COPD pathogenesis. In this proposal, we will focus on the underlying cause of impaired mucosal immunobarrier function which we believe to abnormal epithelial differentiation. Our preliminary data demonstrate that the differentiation factor p73, recently shown to be required for the MCC development, is required for MCC-specific expression of a transport protein required for SIgA transcytosis (polymeric immunoglobulin receptor or pIgR). We found that cigarette smoke suppresses p73 expression in vitro and in vivo, providing a mechanistic link between cigarette smoke exposure, abnormal epithelial differentiation, and impaired mucosal immunobarrier function. In this proposal, we will test the hypothesis that cigarette smoke suppresses p73, resulting in loss of MCCs, reduced pIgR expression and SIgA transcytosis, and impaired immunobarrier function. In Aim 1, we will investigate the impact of targeted deletion of p73 or pIgR in FoxJ1-expressing multiciliated cells on airway epithelial differentiation and immune defense. In Aim 2 we will identify p73-dependent transcription factors that regulate pIgR in murine tracheal epithelial cells and validate our findings in primary human bronchial epithelial cells. In Aim 3 we will define the relationships between cigarette smoke, p73 expression, and epithelial differentiation in vivo. Together, these studies will determine how defects in normal epithelial differentiation result in alterations in mucosal immunity and evaluate whether p73 represents a mechanistic link between chronic inflammation and MCC loss.